rebreathers on Subsurface

Jef Driesen jef at
Fri May 23 06:13:05 PDT 2014

On 2014-05-20 09:29, Dirk Hohndel wrote:
> Correct. And and important factor here is "how do dive computers report
> this". I think Jef commented on this a little while ago - no one 
> appears
> to have seen a dive computer that actually can report data on two
> cylinders. Do you have data from CCR divers that shows how this is 
> done?

A small correction: a dive computer that actually can report data on two 
or more cylinders *simultaneously* per sample. I know that's what you 
meant, but others people may miss this subtle but very important detail 

> Please explain more what you think could destroy this relationship? 
> Right
> now we have the frustrating reality that most dive computers cannot 
> deal
> with multiple cylinders using the same gas (and some don't even allow 
> you
> to enter more than one cylinder with the same gas) because they don't
> report switches between cylinders but switches between gases. We 
> adopted
> that model from libdivecomputer and therefore have some flexibility
> regarding the association of a pressure graph with the corresponding
> cylinder, but I would actually like to modify this specifically in 
> order
> for people to have more than one cylinder with the same gas and to 
> switch
> between them (this is a common request we get from side mount divers, 
> for
> example). So whatever new design we come up with needs to keep that
> direction in mind as well.

For a dive computer it makes sense to manage the tank information 
separately from the gas mixes. Obviously, from a practical point of view 
each gas mix needs to be stored in some tank. But for a dive computer, 
that's pretty much irrelevant in most cases.

First of all, for the decompression calculations, only the available gas 
mixes really matter. The tank sensor is basically only used to show the 
tank pressure and estimate your remaining airtime. In theory a dive 
computer could use it to influence the decompression to some extend 
(e.g. automatically propose a gas change when a tank is almost empty), 
but I doubt this is something that is done in practice.

Last but not least, many dive computers do not have any air integration 
at all. And the ones that do, do not necessary have a tank sensor 
attached to all the tanks you're carrying. For example a hosed dive 
computer (Cobalt, Cobra, etc) can support only a single tank pressure 
(e.g. the one connect to the cable), although many of them do support 
more than one gas mix. The opposite is also possible. There are systems 
that support monitoring the tank pressure of your buddy. In that case 
you have tank pressure, but no gas mix data. And it's not difficult to 
imagine other scenario's...

The important message here is that there is not always a nice one to one 
mapping between tanks and gas mixes. Logically this mapping always 
exists, but in practice the dive computer simply doesn't provide us 
enough data to figure this out correctly. And you can't really blame the 
dive computer, because it simply doesn't have (or need) this kind of 

That's why libdivecomputer doesn't even try to do this. You can probably 
get some of the common and easy cases right (e.g. one tank and one gas 
mix), but everything else is very tricky. The only one that can do this 
correctly is the end-user.


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